In a variety of machinery, working members are driven by hydraulic cylinders which are controlled by a reciprocal spool control valve. Such hydraulic systems are used to control multiple functions, such as raising and lowering the member, tilting or rotating the member around about an axis, and sliding the member fore and aft.
The speed of the hydraulically driven working member depends upon the cross-sectional area of principal narrowed orifices in the hydraulic system and the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to set and maintain the pressure drop. These previous control systems include load sense (LS) lines which transmit the pressure at the valve workports to a control port of a variable displacement hydraulic pump which supplies pressurized hydraulic fluid in the system. In a multi-valve system the greatest workport pressure among the valves is applied to control the pump. The output pressure from this type of pump is the pressure at control port plus a constant pressure, known as the "margin." Thus the displacement of the pump varies in response to changes in the workport pressures that results from the loads on the working members.
Each valve section has a pressure compensator that responds to the pump output pressure and the control port pressure to ensure that the margin pressure appears across the valve's metering orifice even as the pump output pressure varies with load changes. Thus an approximately constant pressure drop is provided across the metering orifice whose cross-sectional area is controlled by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of movement of the working member is determined only by the cross-sectional area of the orifice. This type of system is disclosed in U.S. Pat. No. 5,791,142 entitled "Hydraulic Control Valve System with Split Pressure Compensator", the disclosure of which is incorporated herein by reference.
Although the pressure compensator normally ensures that the pump's margin pressure appears across each valve, it is possible in some hydraulic systems to have a lower pressure across selected valves to reduce the flow. For example when controlling a hydraulic motor, the operator may wish to place the corresponding valve section in a fully open, detented position to provide continuous flow. However, it may be desired to limit that continuous flow to a rate less than that provided by the fully open position. Therefore, there is a need in some hydraulic systems to provide a compensated pressure differential across a metering orifice in selected valves which is less than the pump margin pressure.
Because the pressure compensator provides a constant flow, as determined by the metering area, the typical method to limit the flow rate is to put an additional orifice in series with the metering spool. This other orifice may be fixed to define the maximum flow or it may be adjustable so that the operator can select a desired flow. Another technique, with a spring operated pressure compensator, is to adjust the spring load mechanically while leaving the metering area constant. Both of these conventional methods require substantial mechanical devices with severe limitations as to their location in the valve assembly. Both also require sizeable springs to handle the relatively large loads that acts on them. The type of compensator referred to in the patent referenced above does not derive its control from a spring, but instead from a hydraulic signal, thus restricting the control options.